1. Field of the Invention.
The present invention relates to the vulcanization of neoprene rubber by using a binary accelerator system in conjunction with metal oxides and to vulcanizates containing the same.
2. Description of the Prior Art.
Neoprene is a type of synthetic rubber based on polymers of chloroprene, i.e., polymers of 2-chloro-1,3-butadiene and includes copolymers of chloroprene with dienes or vinyl compounds in which chloroprene constitutes the predominant monomer. In practice, neoprene is grouped into two classes: "Sulfur modified" and "non-sulfur modified" (U.S. Pat. No. 3,370,051 which is incorporated herein by reference).
Generally, the sulfur modified neoprene requires only metal oxides for vulcanization. Conversely, the non-sulfur modified neoprene cures slowly with metal oxide curatives and a vulcanization accelerator must be added to effect suitable curing characteristics. In the past, a number of accelerators have been used as for example thioureas, triazines, imidazolidine-2-thiones, dithiocarbamic acid esters and others. On industrial scale, ethylenethiourea (2-mercaptoimidazoline) has been widely accepted as an all-purpose accelerator for neoprene. However, recently this material has been found to be carcinogenic and thus, for occupational safety reasons, there is a need for a safer alternative.
It is also known to utilize 3,5-disubstituted tetrahydro-2H-,1,3,5-thiadiazine-2-thiones as neoprene acceleratore (British Pat. No. 956,659). However, the vulcanization rate is relatively slow and other processing characteristics such as scorch resistance are not entirely satisfactory. Since the physical properties of he vulcanizate are related to its state of vulcanization, the vulcanization process had to be improved by increasing the vulcanization rate and/or the scorch delay period. In the past, this has been accomplished by adding a second accelerator of the thiourea type including ethylenethiourea to the thidadazine-2-thione accelerators (German Offfenlegungsschrift No. 2,227,338).
According to the present invention, a vulcanization accelerator for neoprene has been provided which contains no known carcinogens such as the above ethylenethiourea and yet provides vulcanizable compositions with balanced processing and vulcanization characteristics.